1. Field of the Invention
The present invention relates to a drilling tool that can be used for drilling of short-radius deviated wells. In particular, the invention relates to a drilling tool with a flexible drill shaft.
2. Description of the Related Art
In the drilling of oil wells or the like, deviation of the direction of drilling is normally achieved by using a bent housing in the bottom hole assembly (BHA) together with a downhole motor to rotate the drill bit while weight is applied from the surface without rotating the drill string. Alternatively, a rotary steerable system such as the Power Drive system of Schlumberger can be used. Moveable stabilizers are operated from the BHA according to the rotational position of the BHA in the well so as to urge the drill bit in the desired direction. The flexibility in normal steel drill pipe is such that deviations with radius of 150 m can be achieved using these techniques.
Coiled tubing can also be used for drilling applications. In such uses a directional drilling BHA is connected to the end of the coiled tubing. One particular tool is the VIPER Coiled Tubing Drilling System (described in Hill D, Nerne E, Ehlig-Economides C, and Mollinedo M “Reentry Drilling Gives New Life to Aging Fields,” Oilfield Review (Autumn 1996) 4-14) which comprises a drilling head module with connectors for a wireline cable, a logging tool including an number of sensors and associated electronics, an orienting tool including a motor and power electronics, and an drilling unit with a steerable motor. While the system is provided with power and data via a cable, it is also necessary to provide a coiled tubing to push the tool along the well.
One particular use of such drilling tools, is that of re-entry drilling in which further drilling operations are conducted in an existing well for the purposes of improving production, remediation, etc. A review of such techniques can be found in the Hill et al paper referenced above and in SPE 57459 Coiled Tubing Ultrashort-Radius Horizontal Drilling in a Gas Storage Reservoir: A Case Study; E. Kevin Stiles, Mark W. DeRoeun, I. Jason Terry, Steven P. Cornell, Sid J. DuPuy. By using a double articulated, it was possible in this case to achieve a build rate of 65° per 100 ft with short sections (5 ft) showing build rates of 100° per ft. Starting in a 5½ inch “vertical” casing, it was possible to reach horizontal in about 100 ft of vertical depth. It has been possible to achieve deviations of 15 m radius using such techniques.
All of the systems described above have physical limitations on the degree of curvature that can be obtained. When attempting to drill out of a cased hole, this means that it is necessary to mill an elongated hole in the casing for the BHA to be able to pass through into the formation around the borehole. Also, the amount of curvature that can be obtained is highly dependent on the type of rock in the formation.
Other techniques have been proposed for drilling laterally from an existing well.
U.S. Pat. No. 6,276,453 discloses a drilling tool including a drill shaft comprising a series of discs which can be guided along a curved path so as to extend laterally from a borehole and to transmit percussion forces to a drill bit at the end thereof. This technique is not applicable to rotary drilling and it is not possible to withdraw the shaft from the hole after drilling.
U.S. Pat. Nos. 5,687,806 and 6,167,968 describe a drilling system in which a flexible shaft is used to provide torque to a drill bit and a thrust support causes weight to be applied to the drill bit and to drive the bit a short way into the formation from the borehole. The diameter of the hole drilled and its extent into the formation are small and unsuitable for production of fluids or placement of measurement devices.